There are many known techniques for nondestructively examining a test specimen, such as a turbine blade. Many of these techniques involve introducing energy into the specimen and detecting a modified form of that energy as it leaves the specimen. For example, it is known to apply X-ray, ultrasonic, magnetic, or heat energy into a test specimen and to detect flaws in the specimen as perturbations in the respective energy pattern as it returns from the specimen.
Acoustic thermography is one such form of nondestructive examination that involves the application of acoustic energy to a test specimen and the measurement of heat energy that is generated within the specimen as a result of the acoustic energy interacting with a flaw, e.g., a crack. Under one of several plausible physical mechanisms for heating, it is believed that as acoustic energy passes through the material of a specimen, opposing surfaces of a crack or other flaw may rub together, thus generating heat. Because undamaged areas of the specimen are only minimally heated by the acoustic waves, a thermal image of the specimen will reveal the flawed area as exhibiting an increase in temperature.
The effectiveness of an acoustic thermography examination is related to the efficiency of the input of acoustic energy into the test specimen. Thus, a means for quickly and reliably determining a sufficient amount of energy transfer into the specimen is desired. Although accelerometers and laser-based vibrometers are accurate devices for determining the amount of acoustic energy applied to the specimen, undesirably, such devices tend to be expensive, cumbersome, and time-consuming for deployment in industrial applications.
In known acoustic thermography techniques, it has been observed that just relatively tight flaws appropriately heat up to become detectable. That is, relatively wide, open voids or cracks may not sufficiently heat up under known acoustic thermography techniques. It would be desirable to provide relatively inexpensive techniques that would systematically allow for these wide voids or flaws to become visible during acoustic thermography.